1. Field of the Invention
The field of the present invention is hydraulic braking pressure control systems for rear wheel brake, of a type comprising an output chamber defined in a housing to lead to a rear wheel brake cylinder, a control chamber defined in the housing and capable of being connected to a control hydraulic pressure source and a reservoir through a switchover valve means capable of switching the communication and interruption, a control piston being provided to have opposite end faces thereof facing the output chamber and the control chamber, respectively, a partition wall provided in the housing for partitioning the output chamber and an input chamber which is defined in the housing to lead to a master cylinder, and a cut-off valve interposed between the input and output chambers and adapted to be closed in response to movement of the control piston toward the control chamber.
2. Description of the Prior Art
Such a hydraulic braking pressure control system is conventionally known, for example, from Japanese Patent Application Laid-open No. 222354/85.
In such a hydraulic braking pressure control system, it is necessary to previously cope with a case in which a defect or fall may occur in control hydraulic pressure from the control hydraulic pressure source for any reason. In the above prior art, when such a defect in control hydraulic pressure occurs, the cut-off valve is operated to open, permitting a hydraulic braking pressure from the master cylinder to be applied to the brake device.
However, since the control piston is permitted to move in a direction to reduce the volume of the control chamber, in other words, in a direction to increase the volume of the output chamber when the control hydraulic pressure source is in trouble, it is difficult to increase the hydraulic braking pressure applied to the brake cylinder. In view of this, a hydraulic braking pressure control system has been proposed in Japanese Patent Application Laid-open No. 107435/85, in which the control chamber is brought into a tightly closed state when a defect in control hydraulic pressure happens. In this system, however, when the control hydraulic pressure is lost, the cut-off valve is permitted to be closed and a passage bypassing the cut-off valve is opened so as to connect the input and output chambers. The system requires such a passage and means for opening and closing the passage in addition to the cut-off valve, resulting in a complicated construction.
Thereupon, the present assignee has already proposed a hydraulic braking pressure control system as shown in Japanese Utility Model Application No. 132318/89, which has a simple construction to open the cut-off valve and to close the control chamber tightly when the control hydraulic pressure is lost.
In a brake device for a passenger vehicle, it is usual to reduce the braking pressure for a rear wheel, as compared with the braking pressure for a front wheel, in order to enhance the braking effect. For providing such a reduction in the braking pressure for a rear wheel, a proportional pressure-reducing valve is interposed between the hydraulic braking pressure control system and a rear brake wheel cylinder. If it is possible to provide the hydraulic braking pressure control system with the function of the proportional pressure-reducing valve, the valve will be unnecessary, and this will contribute to a reduction in number of parts.